Method of treating cathode surfaces in alumina reduction cells



United States Patent 3,471,380 METHOD OF TREATING CATHODE SURFACES 1N ALUMINA REDUCTION CELLS Vaughn L. Bullough, Florence, Ala., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Filed Oct. 25, 1966, Ser. No. 589,215 Int. Cl. C22d1/06, 1/08, 3/12 US. Cl. 204-67 11 Claims ABSTRACT OF THE DISCLOSURE Method of operating an alumina reduction cell to form a beneficial metal carbide on the carbon cathode surface of the cell. The method is described in terms of adding to the electrolyte or the carbonaceous cathode material of the cell a refractory metal (or compound thereof) which, during operation of the cell, is effective to form in situ a carbide of said metal.

This invention relates to a novel method for improving the operation of alumina reduction cells. More particularly, the invention concerns the addition to the cell feed or to the molten bath, of a refractory metal or a compound of such metal.

Much work has been done in an effort to improve the cathode performance of alumina reduction cells, and particularly to lower the overall cell voltage. It is known that titanium carbide is strongly wetted by molten aluminum, as is also zirconium carbide. This property, together with other desirable characteristics, such as good electrical conductivity and low solubility in molten aluminum, have led to the use of carbides and borides of titanium and zirconium as cathode elements, particularly as current leads. However, these compounds are very expensive and must be fabricated by powder metallurgy methods, factors which have limited their widespread adoption.

In accordance with the present invention, it was found, surprisingly and unexpectedly, that by the inclusion of a metal of Groups IV-B and V-B of the periodic system of the elements in the cell feed or as an additive to the molten cell electrolyte, either as such or as a compound of the metal, the metal will deposit on the carbon cathode cell lining, forming in situ a layer of the metal carbide on the cathode surface.

It was also found, in accordance with the invention, that the refractory metal could be supplied to the alumina reduction cell in the form of an aluminous ore, such as bauxite, which is high in the content of the metal. Such aluminous ore can be employed as cell feed per se, or in combination with conventional Bayer alumina feed.

Metals of Group -IV-B which may be employed for the purposes of the invention includes titanium, zirconium, and hafnium, while metals of Group V-B include vanadium, columbium and tantalum. The preferred metal is titanium, and this metal and its compounds will be used for purposes of illustration, it being understood that the other metals named are also applicable for purposes of the invention.

In performing the method of the invention, the foregoing metals, or their intermetallic compounds, or their chemical compounds, are supplied to the alumina reduction cell in amounts and the cell is operated in the presence of these metals, for a period of time sufficient to form a metal carbide coating electrolytically on the cell cathode surface. The formation of the desired carbide coating is indicated by a sharp reduction in the cell voltage, which reduction may amount to as much as 0.5 volt.

The practice of the invention contemplates operation of the cell only for a period of time suflicient to form the metal carbide cathode coating during the initial operation of the cell. Thereafter the cell is returned to regular alumina feed, although the voltage reduction eflect continues for a considerable time after the additive metal feed is discontinued. Similarly, old alumina reduction cells can be reconditioned to reduce excessively high cathode voltage drops, by feeding to the cell the additive metal or its compound or by the addition to the electrolyte of the metal or an intermetallic compound or alloy thereof with aluminum.

In accordance with another aspect of the invention, cathodes manufactured from carbon blacks or graphite can be put into operation in the cell on a molten bath containing the additive metal or its alloys or compounds with aluminum. Alternatively, carbon cell linings can be manufactured which contain the additive metal, or an alloy or intermetallic compound thereof with aluminum, as a component of the carbon mix. The metal then becomes converted to its carbide during baking or during early cell operation.

While not wishing to be bound by any particular theory, it is believed that the voltage reduction is attributable to codeposition of the additive metal, such as titanium, along with aluminum, and that the titanium reacts with the carbon surface at the cathode to reduce the contact film between the carbon and the aluminum. In old aluminum cell cathodes it is probable that aluminum carbide formed on the cathode surface is converted to titanium carbide.

As indicated previously, the additive metal may be introduced as such, or as an alloy or intermetallic compound, for example, as titanium, titanium-aluminum alloy, or titanium aluminide. The additive metal may also be employed in the form of a chemical compound or an ore, for example, as an oxide.

In accordance with the present invention, it has been found that the formation of a titanium carbide coating on the cell cathode is facilitated by incorporating in the cell feed during initial operation of the cell, as much as of an aluminous ore high in titanium content, such as British Guiana bauxite. This accomplishes a reduction in cell voltage of as much as /2 volt, while at the same time titanium carbide is formed electrolytically on the cathode surface.

A typical composition of British Guiana bauxite is as follows:

As dried, wt. As calcined, wt. Component percent percent A1203 61. 12 87. 50 F8203 1. 50 2. 14 SiO 5. 02 7. 16 T10; 2. 50 3. 58 Loss on ignition 30. 02

EXAMPLE An alumina reduction cell of IOU-ampere capacity was cycled through a series of experiments in which the cell was started, operated for several hours and then shut down. After eight such cycles, the cell voltage had stabilized at about 6 volts and had remained constant over five experimental cycles. During the tenth cycle, British Guiana bauxite, of the analysis reported above, was substituted for the normal Bayer alumina used to the extent of about 5 wt. percent of the electrolyte. During this cycle, the cell voltage dropped to about 5.5 volts at the same fixed anode-cathode distance that had been used on all previous cycles. Five additional experimental cycles were conducted in which the cell was returned to the conventional Bayer alumina used in the first nine cycles. During these cycles, the cell voltage remained low and was still below 5.8 volts. At this point, the experiment was terminated.

While the present preferred practices of the invention have been described, it will be appreciated that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. Method of operating an alumina reduction cell in which a carbon cathode surface is disposed in contact with the electrolyte of the cell which comprises operating the cell with a modified electrolyte containing a member selected from the group consisting of the metals of Groups IVB and VB of the periodic system and compounds of such metals, to form a carbide of said metal on the carbon surface; and then proceeding with normal operation of the cell, replenishing the electrolyte with alumina.

2. The method of claim 1 in which the metal is titanium.

3. The method of claim 1 in which the modified electrolyte comprises an aluminous ore which is high in titanium content.

4. The method of claim 3 in which said aluminous ore is British Guiana bauxite.

5. The method of claim 1 in which titanium carbide is formed on a carbon cathode surface of the cell.

6. The method of improving the operation of an alumina reduction cell having a carbonaceous cathode lining which comprises incorporating in said lining of the cell a member selected from the group consisting of the metals of Groups IVB and VB of the periodic system and compounds of such metals, and operating the cell to convert such member to a carbide of said metal.

7. The method of claim 6 in which the metal is titanium.

8. The method of reconditioning the carbonaceous cathode lining of an alumina reduction cell to remove aluminum carbide from the surface thereof which comprises temporarily operating the cell with a modified electrolyte containing a member selected from the group consisting of the metals of Groups IVB and VB of the periodic system and compounds of such metals, to convert said aluminum carbide to a carbide of said metal.

9. The method of claim 8 in which the modified electrolyte comprises an aluminous ore high in titanium content.

10. The method of claim 9 in which said aluminous ore is British Guiana bauxite.

11. The method of claim 8 in which the metal is titanium.

References Cited UNITED STATES PATENTS 400,666 4/1889 Hall 20467 795,886 8/1905 Betts 20467 2,451,492 10/1948 Johnson 20467 3,400,061 9/1968 Lewis et al. 20467 HOWARD S. WILLIAMS, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 

